1. Field of the Invention
The present invention relates to a device for determining cardiac function parameters by analyzing the intracardial impedance curve. For this purpose, the device has an impedance measuring unit which has electrical terminals to which electrodes for delivering a current and for detecting a voltage are to be connected or are permanently connected. Furthermore, the device has an analysis unit, which is connected to the impedance measuring unit and is implemented in such a way that it may derive a cardiac function parameter from a time curve of the impedance ascertained using the impedance measuring unit.
2. Description of the Related Art
A suitable impedance measuring system is known, for example, from German patent application DE 103 61 143. The impedance measuring system described therein is implemented to ascertain a cardiac function parameter proportional to the stroke volume of the heart from the intracardially detected impedance, namely the stroke impedance. The stroke volume describes the blood volume which is delivered by the heart during an ejection phase (systole). To ascertain the stroke impedance, the difference between the final diastolic impedance (at the end of the filling phase, i.e., diastole, of the heart) and the final systolic impedance at the end of the ejection phase (systole) is ascertained. Since the intracardially detected impedance is decisively determined by the good conductivity of the blood filling the heart, the impedance when the heart is maximally filled at the end of the diastole is less than when the ventricle is maximally contracted at the end of the systole. The final diastolic impedance is more or less coincident with the minimum of the impedance curve during a cardiac cycle, while the final systolic impedance is the maximum of the impedance curve during a cardiac cycle.
An impedance measuring system which is also preferred for the present invention is a quadropolar system as shown in FIG. 1a of DE 103 61 143 and described in the associated description. In a preferred impedance measuring system of this type, a pulsed constant current is delivered between the terminal for a right-ventricular tip electrode and the terminal for a right-ventricular ring electrode. In this preferred case, the voltage caused by the current is measured between the terminal for a left-ventricular tip electrode and the terminal for a left-ventricular ring electrode.